The present invention relates to a coordinate reading system for inputting coordinates to an external device such as a computer and, more particularly, to an electromagnetic induction type coordinate system which makes use of electromagnetic induction phenomena.
Coordinate reading systems of the type already known in the prior art are exemplified by commonly-owned Japanese Laid-Open Patent application Nos. 96825/1977 and 96411/1980.
An exemplary coordinate reading system according to the prior art will be briefly described in the following.
FIG. 8 is a diagram showing the structure of the exemplary coordinate reading system of the prior art. In a tablet acting as a coordinate reading plate, there are laid a plurality of sense line groups 101 which have loop shapes. The tablet itself is not explicitly shown in FIG. 8. The sense line groups are selected one by one by a scanning circuit 102. This scanning circuit 102 has its output connected to a signal processing circuit 105, the output of which in turn is connected to a control circuit 106. This control circuit 106 feeds a scanning signal s101 to cause the aforementioned scanning circuit 102 to scan groups 101 sequentially.
The system further includes a coordinate indicator 107 having a coil 107a packaged therein and connected through an excitation signal line 108 to an excitation signal generating circuit 104 which is packaged in the tablet.
With the structure as thus described, coordinate values indicated by indicator 107 are calculated in the following manner. The coordinate indicator 107 is placed on the sense line groups 101 and generates an AC magnetic field at all times in response to the signal from the excitation signal generating circuit 104. The control circuit 106 feeds the scanning signal s101 to close the switches of scanning circuit 102 sequentially. The current induced in each sense line group 101 is dependent upon the position of coordinate indicator 107, and an induction signal s102 for each sense line is sequentially inputted to signal processing circuit 105 by selecting the sense line groups 101 with the scanning circuit 102.
The signal processing circuit 105 generates an envelope waveform of those induction signals. The control circuit 106 determines the coordinate values of the position of indicator 107 by inputting the amplitudes of the induction signals for the individual sense lines from the envelope waveform and by comparing the amplitudes arithmetically.
In this prior art coordinate reading system, however, an AC signal has to be fed to coordinate indicator 107 so as to generate a magnetic field which emanates from the coordinate indicator. For this purpose, the tablet and the coordinate indicator have to be connected through a signal line.